As a method for sensing the presence/absence of a trace substance, for example, an environmental pollutant such as dioxin or a disease marker such as a hepatitis C virus and C-reactive protein (CRP), in a sample solution, and for measuring the concentration of these substances, there has been widely known a measurement method using: a quartz sensor which includes a piezoelectric resonator, for example, a quartz resonator; and a measuring device which is electrically connected to the quartz sensor and includes an oscillator circuit or the like for oscillating the quartz resonator.
Concretely, in the measurement method, the quartz sensor including the quartz resonator called a Langevin-type or the like which includes: a quartz piece being, for example, a plate-shaped piezoelectric piece; and a pair of foil-shaped electrodes for excitation (excitation electrodes) provided on one surface and the other surface of the quartz piece so as to sandwich the quartz piece is structured such that the electrode on the one surface is in contact with a measurement atmosphere (sample solution) and the electrode on the other surface faces an airtight space, and the method utilizes such a property that when a substance to be measured in the sample solution comes into contact with the electrode on the one surface, the natural frequency of the quartz piece changes according to a mass of the substance which is in contact with the electrode.
The reason why the quartz resonator is structured such that only the one surface thereof is in contact with the measurement atmosphere and the other surface faces the airtight space as described above is because this structure is preferable for stable oscillation of the quartz resonator. Generally, an adsorption layer on whose front surface, for example, an antibody is attached is provided on the electrode on the one surface of the quartz resonator. This antibody selectively adsorbs one of the substances to be measured, for example, described above, by an antigen-antibody reaction, and when the substance to be measured is adsorbed by the adsorption layer, the frequency of the quartz piece changes according to an adsorption amount of the substance to be measured. However, when this measurement method is implemented for research, there is a case where the one surface of the quartz resonator is in contact with the measurement atmosphere without the adsorption layer being provided on the front surface of the quartz resonator, and the antibody is physically made to adhere to the electrode of the quartz resonator for the purpose of, for example, analyzing how the antibody adheres to the substance to be measured.
FIG. 15 shows an example of the structure of the vicinity of the quartz resonator provided in the aforesaid quartz sensor. In the drawing, 11 denotes a wiring board, and the quartz resonator 12 is placed on the wiring board 11. The excitation electrodes, not shown, provided on the front and rear surfaces of the quartz resonator 12 are electrically connected to electrodes provided on the wiring board 11, so that the resonator 12 is electrically connected to the wiring board 11.
In the drawing, 13 denotes a through hole bored in the wiring board 11 in its thickness direction, and in the drawing, 14 denotes a sealing member covering the through hole 13 from a rear surface side of the board 11. A region surrounded by these sealing member 14, through hole 11, and quartz resonator 12 forms an airtight space, and the excitation electrode on the rear surface of the quartz resonator 12 faces the airtight space. In the drawing, 15 denotes a quartz pressing member in a plate shape made of, for example, rubber or the like and it presses the quartz resonator 12 toward the board 11 to fix the position of the quartz resonator 12.
In the drawing, 16 denotes an opening portion provided to penetrate through the quartz pressing member 15 in the thickness direction and it faces the excitation electrode on the front surface of the quartz resonator 12. In the drawing, 17 denotes an annular projection of the quartz pressing member 15, which will be described later. A predetermined amount of the sample solution is stored in a solution storage space 18 surrounded by the opening portion 16 and the annular projection 17, so that the excitation electrode comes into contact with the measurement atmosphere.
In the quartz sensor as described above, a large stress, if applied to the quartz resonator 12 by the quartz pressing member 15, hinders the oscillation of the quartz resonator 12, and the smaller the stress is, the more stably the quartz resonator 12 oscillates to enable higher-precision measurement. Therefore, it has been considered to reduce the stress by decreasing an area of a portion, of the quartz pressing member 15, that is in contact with and presses the quartz resonator 12. As a concrete example of this, it has been considered to form the annular projection 17 on a rear surface of the pressing member 15 so as to surround the periphery of the opening portion 16 as shown in the drawing and press the quartz resonator 12 by a tip portion of the projection 17 toward the wiring board 11 to fix the position of the quartz resonator 12.
However, with such a structure, bubbles 19 are sometime formed from air existing in the solution storage space 18 and air mixed in the sample solution when, for example, the sample solution is supplied to the solution storage space 18. In the solution storage space 18, a pressure is applied toward a corner portion made by an outwardly/downwardly inclined peripheral side surface of the projection 17 and the front surface of the quartz resonator 12 as shown by the arrows in the drawing, and therefore, the bubbles 19 sometimes enter the corner portion. At this time, since the inner peripheral side surface of the projection 17 is above the bubbles 19, this surface may possibly prevent the bubbles 19 from floating up to shut the bubbles 19 in the corner portion. In this case, an amount of the sample solution 18 stored in the solution storage space 18 changes by a volume of the bubbles 19, and being influenced by this, an amount of the substance to be measured in the solution storage space 18 also changes. This is likely to cause a measurement error, and especially because this measurement method is used to sense a trace substance as previously described, such a slight change in the amount of the sample solution may possibly cause an error, for example, a substance that should be able to be detected cannot be detected.
A patent document 1 and a patent document 2 also describe quartz sensors utilizing a Languban-type quartz resonator, but neither of them solves the aforesaid problems.    Patent Document 1: Japanese Patent Application Laid-open No. 2006-029873 (paragraph 0020, paragraph 0021, and FIG. 1)    Patent Document 2: Japanese Patent Application Laid-open No. Hei 11-183479 (FIG. 2)